Devices that operate on an electrical network, e.g., of a manufacturing plant or other installation, consume active and reactive power. The active power is converted into work or heat and may be measured in units of power, such as kilowatts (kW). The reactive power results from a phase shift between the voltage and current, and may be expressed in units of reactive power, such as volt-amperes reactive (var) or kilovolt-amperes reactive (kvar).
Consumption of reactive power is a characteristic of electric devices that utilize the inductive properties of an alternating electromagnetic field, such as motors or transformers. The total electrical power that is consumed by a device may be represented by a vector sum or difference of the reactive and active power components. The magnitude of this vector sum is referred to as apparent power.
This phenomenon of reactive power may have consequences for electrical networks of both low and high voltage. Devices that store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power. Those devices that store energy by virtue of electric fields are said to generate reactive power. Power flows, both active and reactive, must be carefully controlled in order for a power system to operate within acceptable voltage limits. Reactive power flows can give rise to substantial voltage changes across the system. Thus, it is necessary to maintain reactive power balances. Reactive power compensation is an essential feature in a power system's operation and maintenance of acceptable voltage levels during contingencies in power systems. One or more techniques may be applied to compensate for the reactive power of a load that is connected to a power system.
Power electronic devices such as power converters, power supplies, converter-fed motors, and, in some cases, the power compensation circuit itself, such as a static var compensator (SVC), may cause harmonic pollution. Harmonic pollution causes a strong distortion of the fundamental sinusoidal wave shape of voltage and current. Fourier analysis of a fundamental period reveals the presence of typical harmonic frequencies that may be multiples of the fundamental frequency (e.g., 50 Hz or 60 Hz). Harmonic pollution may be caused by power converters and other power electronics that may generate, e.g., in a 50 Hz system, such harmonics as 250 Hz, 350 Hz, 550 Hz, 650 Hz, or higher frequency harmonics.
Energy losses in electrical network elements, such as lines, transformers, or loads, may result from current and harmonic losses. Current losses may include resistive heating in conductive elements. Harmonic losses may include skin effect, hysteresis, and negative sequence losses. The skin effect refers to the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with depth within the interior of the conductor. As a result, the effective cross section of the conductor is reduced and resistive increase. Hysteresis losses may include heat loss that is associated with magnetic hysteresis properties of an AC motor armature or transformer. Negative sequence losses refer to a magnetic field that rotates against the main field in a motor due to fifth, eleventh, or other harmonics, generating heat and reducing efficiency.
Loads may not be stable and may change over time. Some loads may change rapidly, such as those due to molding machines in a plastics plant. Each molding machine has its own cycle of operation and several machines that are fed by a single transformer may operate independently from one another. As a result, power consumption may not be controlled and may change rapidly.
In addition, harmonics in voltage and current may have a substantial effect on power consumption and losses.
For some devices, there are no measurement methods that can directly measure or estimate losses. For example, these devices may include a large number of elements that may be affected by a harmonics spectrum which may change rapidly. The exact relation between harmonics and losses may be unknown, affected by materials, specifics of mechanical structure, and a nonlinear relationship between the harmonics and the losses. Furthermore, a mode of operation of each device may be variable, depending on a particular production process that is in progress.